1. Field of the Invention
The present invention relates to a semiconductor laser element and a semiconductor laser device including the same, and more particularly, to a technology for reducing a beam pitch of a multi-beam semiconductor laser element.
2. Description of the Related Art
For instance, a semiconductor laser device used for a light source of an image printing apparatus such as a laser printer or a copying machine is equipped with a semiconductor laser element (laser chip) in which a light emitting portion is formed on a semiconductor substrate (hereinafter referred to simply as “substrate”). In particular, a multi-beam semiconductor laser element in which a plurality of light emitting portions are formed on a laser chip substrate has an advantage in that the number of scanning beams can be increased to perform high speed printing, and hence demands for the multi-beam semiconductor laser element have rapidly increased.
In view of cost reduction in a manufacturing process, it is desired that the multi-beam semiconductor laser element have a ridge structure. Depending on a semiconductor material used for a laser chip, it is necessary to enhance heat dissipation in order to suppress temperature rise in an active layer. As a mounting method for enhancing the heat dissipation, there is used a junction-down method involving bonding an upper surface of the laser chip (chip surface on a side closer to the active layer) to a submount (support substrate).
As a bonding material for bonding the semiconductor laser element and the submount, solder such as AuSn is usually used. However, because there is a difference in linear expansion coefficient between the semiconductor material of the laser chip and the submount material, thermal stress may occur when the device bonded at high temperature is rapidly cooled to room temperature. In this case, if the thermal stress is applied to the light emitting portion (or a waveguide section) of the semiconductor laser element, polarization property of a laser beam emitted from the semiconductor laser element is changed. As an example, it is known that a polarization plane of the emitted laser beam is rotated so that a polarization angle is increased. If the polarization angle of the laser beam is increased, when an optical component (lens or the like) having polarization angle dependence in optical characteristics is used, there occurs a malfunction such as a variation of a laser beam optical path or transmitted light intensity. JP 2006-278694 A discloses an optical semiconductor device in which uniformity of a reaction layer between a laser chip electrode material and a bonding material is enhanced so that a variation in a polarization direction is suppressed and good polarization property is achieved.
JP 2011-108932 A discloses a semiconductor optical device including a laser diode having a bank structure (bank portion 31) formed on right and left sides of a ridge structure (ridge portion 12). For instance, as illustrated in FIG. 1 of JP 2011-108932 A, a conductor is formed so that a height of an upper surface of the conductor (conductive layer 16) formed on an upper part of the bank portion 31 is higher than a height of an upper surface of the conductor (conductive layer 16) formed on an upper part of the ridge portion 12. Therefore, the bonding material (solder material 20) is melted and bonded to the conductor formed on the upper part of the bank portion 31. In contrast, the bonding material is separated away from the conductor formed on the upper part of the ridge portion 12 via an air gap so as not to be in contact with the conductor. Thus, polarization angle characteristics of the semiconductor optical device are stabilized.